Conventionally, when a heat-insulating structure is used with an automobile or an autobike and subjected to vibration at a temperature of more than 500.degree.C, a ceramic fiber has been used or the method of forming a heat-insulating layer by applying a foamable paste to a heat insulating member and foaming the paste has been adopted.
In a heat-insulating structure using a ceramic fiber, a portion of the structure becomes pulverized after a long period of use with vibration, and cavities are formed in the heat-insulating layer which impair its heat-insulating properties.
Also, in the case of the heat-insulating layer of a manifold reactor, when a ceramic fiber is used, thermal stresses are repeatedly applied due to a temperature variation and the welded portions of the steel structure which hold the heat-insulating layer are broken away and ceramic fibers are sucked through the broken portions into the exhaust gas. This leads to various problems, such as the locking of a bypass valve for the catalytic converter or a valve for the exhaust gas recirculator (E G R).
Also, in the case where a heat-insulating layer is formed by applying a foamable paste to an insulating member and foaming the paste, the layer is subject to peeling when subjected to vibration, with resultant degradation of its heat-insulating properties. While there is a known heat-insulating structure similar to those according to the present invention, which is made by introducing a ceramic paste into an insulating structure (comprising spaced steel walls) and then drying and firing to solidify the ceramic, the ceramic paste is of a low fluidity and thus is incapable of forming a uniform insulating layer when the structure is complicated and the insulating layer is thin, as in the case of an exhaust gas purifier for an automobile. Also, this type of insulating member has a high specific gravity and relatively poor heat insulating properties.